The present invention relates to an engine exhaust purification device provided with a catalyst.
JP-A-H9-228873 published by the Japanese Patent Office in 1997 discloses a technique wherein an oxygen amount stored in a three-way catalyst (hereafter, xe2x80x9coxygen storage amountxe2x80x9d) is estimated based on an engine intake air amount and an air fuel ratio of an exhaust flowing into the catalyst, and engine air-fuel ratio control is performed so that the oxygen storage amount of the catalyst is constant.
To maintain the NOx (nitrogen oxides), CO and HC (hydrocarbon) conversion efficiency of the three-way catalyst at a maximum, the catalyst atmosphere must be maintained at the stoichiometric air-fuel ratio. If the oxygen storage amount of the catalyst is maintained constant, oxygen in the exhaust is stored in the catalyst even if the air-fuel ratio of the exhaust flowing into the catalyst temporarily becomes lean, and conversely, oxygen stored in the catalyst is released even if the air-fuel ratio of the exhaust flowing into the catalyst temporarily becomes rich, so the catalyst atmosphere can be maintained at the stoichiometric air-fuel ratio.
A structure of a second three-way catalyst is disposed downstream of the above three-way catalyst is proposed to satisfy more stringent exhaust emission performance requirements. In this structure, although oxygen is absorbed by the catalysts up to a maximum limit exceeding a required amount after a lean running such as during fuel cut control, only the oxygen storage amount of the upstream catalyst is controlled so that it returns to the required storage amount in subsequent air-fuel ratio control, whereas the second three-way catalyst is not controlled as it is. The oxygen storage amount of the second three-way catalyst is therefore effectively maintained at its maximum, consequently, the conversion efficiency of the second three-way catalyst cannot be maintained at a maximum, and there is a risk that the NOx discharge amount will increase.
It is therefore an object of this invention to resolve the above problem, and provide an engine exhaust purification device wherein the conversion efficiency of a downstream catalyst is maintained at a high-level after lean running.
In order to achieve above object, this invention provides an exhaust purification device comprising a first catalyst provided in an engine exhaust pipe, a second catalyst provided in downstream of the first catalyst, a front sensor which detects an exhaust characteristic flowing into the first catalyst, a first rear sensor which detects an exhaust characteristic flowing out of the first catalyst, a second rear sensor which detects an exhaust characteristic flowing out of the second catalyst, a microprocessor programmed to compute an oxygen storage amount of the first catalyst using the detected exhaust characteristic, to control an air-fuel ratio of the engine so that the oxygen storage amount of the first catalyst is a target amount based on the computed oxygen storage amount, and to control the air-fuel ratio using an exhaust characteristic detected by the second rear sensor so that the oxygen storage amounts of the first catalyst and the second catalyst are a target amount, after running at a lean air-fuel ratio.
This invention further provides an exhaust purification device comprising a first catalyst provided in an exhaust purification device comprising a first catalyst provided in an engine exhaust pipe, a second catalyst provided in downstream of the first catalyst, a front sensor which detects an exhaust characteristic flowing into the first catalyst, a first rear sensor which detects an exhaust characteristic flowing out of the first catalyst, a second rear sensor which detects an exhaust characteristic flowing out of the second catalyst, and a microprocessor programmed to compute the oxygen storage amount stored in the first catalyst using the exhaust characteristic detected by the front sensor and the exhaust characteristic detected by the first rear sensor during normal running, to compute the oxygen storage amount stored in the first catalyst and second catalyst using the exhaust characteristic detected by the front sensor and the exhaust characteristic detected by the second rear sensor after running at a lean air-fuel ratio, control the engine air-fuel ratio so that, during normal running, the oxygen storage amount of the first catalyst is a target amount based on the computed oxygen storage amount in the first catalyst, and to control the engine air-fuel ratio so that, after running at a lean air-fuel ratio, the oxygen storage amount stored in the first catalyst and the second catalyst is a target amount based on the computed oxygen amount in the first catalyst and the second catalyst.
The details as well as other features and advantages of this invention are set forth in the remainder of the specification and are shown in the accompanying drawings.
Strictly speaking, noble metals adsorb oxygen in the molecular state, and oxygen storage materials absorb oxygen as compounds, but in the following description, adsorption and absorption will be collectively referred to as storage.
Further, the expression xe2x80x9cthe exhaust air-fuel ratio is richxe2x80x9d means that the oxygen concentration in the exhaust is lower than the oxygen concentration in the exhaust when the engine is running at the stoichiometric air-fuel ratio, and the expression xe2x80x9cthe exhaust air-fuel ratio is leanxe2x80x9d means that the oxygen concentration in the exhaust is higher than the oxygen concentration in the exhaust when the engine is running at the stoichiometric air-fuel ratio. The expression xe2x80x9cthe exhaust air-fuel ratio is stoichiometricxe2x80x9d means that the oxygen concentration of the exhaust is equal to the oxygen concentration in the exhaust when the engine is running at the stoichiometric air-fuel ratio.